1. Field of the Invention
The present invention relates to a diagnostic ultrasound system, or more particularly, to a diagnostic ultrasound system adaptable to tissue Doppler imaging based on an ultrasonic pulsed-wave Doppler technique.
2. Description of the Related Art
In the past, a diagnostic ultrasound system having a tissue Doppler imaging (TDI) feature has been disclosed, for example, in Japanese Patent Laid-Open No. 6-114059 (of which title of the invention is an "ultrasound color Doppler tomography system") proposed by the present applicant. The diagnostic ultrasound system described in the unexamined patent publication has a feature that uses a pulsed-wave Doppler technique and a lowpass filter to detect the motion velocities of tissues including the cardiac muscle and vascular wall, compute various physical volumes relevant to motion on the basis of the motion velocities, and display the results of computation in appropriate modes in color. For detecting the motion velocity of a tissue, since the motion velocity of a tissue is markedly lower than a blood flow velocity, the pulse repetition frequencies (PRF) of transmitted ultrasonic pulsed waves (rate pulses) are lowered to enable measurement of super-low motion velocities of tissues.
Various modes are available for color display of the results of computation. In the invention described in the unexamined patent publication, two-dimensional color display has been proposed. As for the gradations for the color display, a procedure used for the blood flow imaging, which is implemented in a color Doppler system and shares concepts with tissue Doppler imaging, can be employed.
In the blood flow imaging, a band of Doppler shift frequencies fd ranging from -fr/2 to fr/2 (where, fr denotes a pulse repetition frequency of an ultrasonic pulsed wave) is rendered, as shown in FIG. 27, in 32 gradation (gray-scale)levels (fr/32 per level) with different color brightnesses or hues. In other words, a scale whose gradation levels associated with velocities (Doppler shifts) have a constantly progressive change is assigned to the whole band of Doppler shift frequencies ranging from -fr/2 to fr/2, thus defining a color-display gradation between red (yellow) to blue (light blue).
As mentioned above, one of the characteristics of tissue Doppler imaging lies in that pulse repetition frequencies (lower frame rates) of ultrasonic waves are set to lower values in order to enable measurement of ultra-low motion velocities of tissues. Owing to the characteristic of enabling measurement of an ultra-low motion velocity, the band of Doppler shift frequencies required for display images produced by tissue Doppler imaging is narrower than that required for display images produced by blood flow imaging of ranges, for example, from -fr/8 to fr/8.
Nevertheless, at present, the assignment of a color-display gradation adopted for blood flow imaging cannot help applying to tissue Doppler imaging as it is. As a result, the number of gradation levels assigned to a low-velocity band is quite limited. A tissue region to be observed; such as, the cardiac muscle appears, for example, in red of almost the same hue or brightness. It is therefore very hard to visually assess a difference in velocity in a low-velocity band image. Even if a difference in velocity smaller than fr/32 can be detected, since a displayed hue or brightness is unchanged, high-precision detectability is canceled out by poor displaying ability. There still exists an unsolved problem that the high-precision detectability cannot be exerted fully.
When tissue Doppler imaging is used for diagnosis, it should be discerned promptly whether the cardiac muscle or any other tissue region of interest is normal or abnormal. Using the conventional display technique, the levels of a color-display gradation are assigned uniformly between a low-velocity band and a high-velocity band. The displaying ability for the low-velocity band is, as described previously, poor. There is therefore difficulty in discerning whether a diagnostic region is normal or abnormal. Consequently, it takes too much time for diagnosis. Moreover, an examining physician is requested to have high expertise.